The present invention relates to a traffic control system of a local area network (to be referred to as a "LAN" hereinafter) of a Carrier Sense Multiple Access (CSMA) method or a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method.
Conventionally, there are generally used two types of traffic control system of the CSMA method or CSMA/CD method. In the first type of traffic control system, a carrier signal is monitored at data transmission starting so as to avoid the collision of transmission data. The detection of the carrier signal in a LAN, indicates that another terminal is on data transmission and the terminal delays the data transmission by a time period determined based on a random number generated by a random number generator, i.e., suspends the data transmission and monitors a carrier signal again. When any carrier signal is not detected, the data transmission is performed. If any carrier signal is detected after the delay of the data transmission, the delay is performed again and this operation is repeated until no carrier signal is detected.
In the second type of traffic control system, the number of terminals for data transmission which are connected to a LAN is limited, so that a network can be constructed to have a LAN load less than a predetermined level. That is, if the number of terminals connected to the LAN exceeds the limitation, the LAN is divided into a plurality of LANs which are connected to each other via relay units.
In the first type of conventional traffic control system, however, when an amount of transmission data is so much that the network is in the high traffic state, the suspension of data transmission would often occur because there is a high possibility that the other terminals perform data transmission in transmission from the terminal. The terminal which first detects that no carrier signal is present in the LAN is released from the transmission suspending state to come into the transmittable state. Therefore, there are terminals which cannot almost perform the data transmission and terminals which can perform the data transmission with a higher possibility than that of other terminals, so that unbalance between the terminals in data transmission could be caused. When there are data having a higher priority and data having a lower priority, there is another problem in that the data having a higher priority cannot be transmitted when the LAN is in the high load state.
In the second type of traffic control system, terminals are divided in advance in accordance with a plurality of LANs such that the traffic load can be even distributed, so that a hierarchical network can be constructed. However, the network is not always in the high traffic load state. In a case where the network is in the low traffic load state, there is another problem in that transmission delay would be caused because of the relay units. That is, when data should be transmitted between the terminals connected to the different LANs via a relay unit, the hierarchical network gives the opposite effect to what is intended.